1. Field of the Invention
The present invention relates generally to an apparatus that allows characterization of material mechanical and physical properties, and more particularly, but not by way of limitation, to a test cell that is capable of applying a shear stress to a small test sample of material, such as rock, bone, and cartilage, and which incorporates the ability to determine acoustic compressional and shear wave velocities in multiple orientations while the sample is subjected to an axial load and to fluid flow at various pressures and various fluid chemistries.
2. Brief Description of Related Art
Knowing the effects of fluid exposure and the directional dependency of mechanical properties such as strength and elastic/poroelastic coefficients is crucial for well-bore stability analysis, hydraulic fracturing design, and many other field applications in the oil and gas industry. Conventional test cells for loading test specimens apply a generally uniform radial pressure or confining stress and an axial stress. A Hoek cell, for instance, applies axial stress on the two ends of a cylindrical specimen, while the radial stress is developed by pressurizing a hydraulic fluid such as oil, around the cylindrical surface of the specimen, in a test chamber in which the specimen is held. The radial stress is angularly uniform in that it is the same in all radial directions and the only variations possible in relation to differential stress loading are axial and radial (angularly uniform) relative to each other.
Test data from the field has shown that the radial stress, more usually referred to as the horizontal stress, is defined by two principal stresses and is asymmetrical. The horizontal stress applied by a Hoek cell is symmetrical and, as such, not suitable for certain testing, such as well break-out, shear wave splitting, and fracture propagation testing. In order that specimens may be tested with asymmetrical, horizontal stresses, it has been necessary to prepare cubes of test material which much more accurately reflect the principal stresses encountered in an actual three dimensional situation. Such cuboid samples are, however, more difficult and expensive to prepare and test. Furthermore, cuboid samples generally cannot be prepared from the cylindrical test specimens normally obtained by conventional coring techniques used for sample recovery, e.g., in the petroleum industry.
To this end, a need exists for a test cell and method capable of subjecting a test sample to different applied stress states, and fluid circulation for any desired time of exposure, while the dynamic elastic moduli can be simultaneously acquired and monitored applying a shear stress to a test sample while also applying an axial load as well. It is to such a test cell and method that the present invention is directed.